Integrated intensity of continuous absorption in infrared spectra of complexes with medium-strong and strong hydrogen bonds

Abstract

A simple, nongraphical and reproducible method of separation of the complex absorption due to strong hydrogen bonds from that due to the skeleton is proposed. The method has been tested on 17 complexes of pyridine N-oxide, triphenylphosphine oxide and DMSO- d 6 with dichloroacetic acid in dry dichloromethane and acetonitrile. The integrated intensity ( A CPA) and the centre of gravity (mathtype1) of complex absorption due to protonic vibration were measured and correlated with p K a values of bases and chemical shifts of the hydrogen-bonded protons (δ), and discussed with respect to hydrogen bond strength variations. The A CPA values vary from 17.5 to 46 x 10 4 cm mmol -1 and were reproducible to within ± 1 x 10 4 cm mmol −1 (5-15 %). A nonlinear correlation between A CPA and (mathtype2) has been found in wide region of data; (mathtype3) varies from 500 to 2300 cm −1. A gradual proton transfer has been considered from the acid to pyridine N-oxides via strengthening intermolecular H-bonds, AH⋯ON, and further via weakening interionic H-bonds, A −⋯ HON +. The obtained correlations suggest that variations of hydrogen bond strength caused similar changes of A CPA and (mathtype4) both in molecular (A–H⋯B) and ionic (A −⋯H–B +) species.